1. Field of the Invention
The present invention relates to an optical transceiver used in transmitting/receiving optical signals and particularly, to a laser driver IC capable of meeting a variety of specifications without increasing the number of terminals, and an optical transceiver using the laser driver IC.
2. Description of the Related Art
Optical transceivers used in transmitting/receiving optical signals in optical communications are typically detachably mounted to bodies of communication devices. Inside the optical transceivers, there are provided a light-receiving element used for receiving light signals, a circuit for amplifying its received light signals, a light-emitting element used for transmitting light signals, and a drive circuit for driving that light-emitting element with transmitted signals. It is desirable that the optical transceivers have a function of diagnosing faults regarding a laser that is a light-emitting element and a circuit for driving the laser, to force its driving to stop, and a function of indicating faults to a communication device. It is also desirable to have a function of resetting its forced stop state. These functions are collectively called a fault function.
However, there are conventionally two types of optical transceivers: a type of having no fault function, and a type of having a fault function. Which type of optical transceiver to use depends on communication device specifications. Accordingly, manufacturing makers (or distributors) of optical transceivers must prepare shipment for both types of optical transceivers at any time according to demands of customers (sites where communication devices are used).
To ensure that optical transceivers are reduced in size, on the other hand, a circuit for driving a laser is integrated in one IC in the optical transceiver, and a circuit for a fault function (hereinafter, “fault circuit”) also in the same IC. In other words, a circuit for driving a laser element, and a fault circuit for detecting a fault in that laser element are incorporated in a laser driver IC.
As shown in FIG. 1, a fault circuit in a conventional laser driver IC comprises an RS flip-flop 1 for latching a signal that indicates an event of fault detection, to output it as an internal fault signal, and a TTL open-collector type buffer 11 for converting that internal fault signal into a fault signal outside the laser driver IC.
The RS flip-flop 1 has an S-input terminal for inputting a set signal, an R-input terminal for inputting a reset signal, a Q-output terminal for outputting a non-inversion logic, and a Q-bar output terminal for outputting an inversion logic. Input into the S-input terminal is a signal that indicates an event of fault detection. That is, although not illustrated, there are provided a monitor PD (photodiode) facing toward the light-emitting element, and a diagnosis circuit for analyzing an output signal of that monitor PD to diagnose a fault in the light-emitting element, to input an output of that diagnosis circuit directly or indirectly into the S-input terminal. Input directly or indirectly into the R-input terminal is a reset signal from outside the laser driver IC. The Q-output terminal is connected to other circuits not illustrated within the laser driver IC. In these other circuits, an output of the Q-output terminal is used as a disable signal (output #21) for inactivating the driving function of the laser driver IC. The Q-bar output terminal is connected to an input terminal of the buffer 11. The output terminal of the buffer 11 is connected to a fault signal output terminal 12 linked to outside the laser driver IC. Outside the laser driver IC, by adding a pull-up resistor not illustrated, an output of the fault signal output terminal 12 is used as a fault signal (output #22). Accordingly, a fault signal appearing at this fault signal output terminal 12 can be taken out from outside the laser driver IC (inside the optical transceiver) not illustrated. Inside the optical transceiver, this fault signal is conducted directly or indirectly to an external fault signal output terminal (not illustrated), to take an external fault signal indicating a fault to outside the optical transceiver (to a communication device) (See Japanese patent application laid-open No.9-321386).
Table 1 shows a truth table for the circuit of FIG. 1.
TABLE 1SRQ#21#22HLHHHLHLLLHHUnknownUnknownUnknownLLPrecedingPrecedingPrecedingstate heldstate heldstate held
In Table 1, the states of the S-input terminal are shown in the S-column, the states of the R-input terminal in the R-column, the states of the Q-output terminal in the Q-column, the states of the disable signal in the output #21 column, and the states of the fault signal in the output #22 column, while the state of each signal at the same time is shown for each row, where state H denotes a high voltage level, and state L a low voltage level.
As mentioned above, since conventional optical transceivers are classified into those having no fault function, and those having a fault function, communication devices are also divided into those with specifications presupposing that optical transceivers have no fault function, and those with specifications presupposing that optical transceivers have a fault function. For this reason, when manufacturing makers or distributors of optical transceivers make optical transceivers of separate types, they need to select laser driver ICs that match their specifications. If the same laser driver IC can be used regardless of the presence/absence of a fault function of optical transceivers, stock can be saved, while operation efficiency of manufacturing lines is enhanced. To aggregate parts inside an optical transceiver, it is preferred to place the parts in its laser driver IC as much as possible. Further, even if the function of the laser driver IC is increased to ensure the above-mentioned use for both types of optical transceivers, it is preferred not to increase the number of terminals of the laser driver IC from the point of view of aggregating parts.
Here, specifications are classified, dependent on whether a disable state is produced and on whether a fault signal is output, when a light-emitting element and/or a circuit for driving that light-emitting element malfunctions.
TABLE 2Disable stateFault signal outputFirst specificationsYYSecond specificationsYNThird specificationsNYFourth specificationsNN
Table 2 shows the presence/absence of the disable state and fault signal output, where the presence of the disable state and fault signal output is denoted by letter “Y”, and the absence thereof is denoted by letter “N”. According to combinations of the presence and absence of the disable state and fault signal output, four kinds of specifications are considered, as shown in table 2. The first specifications are that both the disable state and fault signal output are obtained when a light-emitting element and/or a circuit for driving that light-emitting element malfunctions. The second specifications are that only the disable state is obtained and no fault signal output obtained when a light-emitting element and/or a circuit for driving that light-emitting element malfunctions. The third specifications are that only the fault signal output is obtained and no disable state obtained when a light-emitting element and/or a circuit for driving that light-emitting element malfunctions. The fourth specifications are that neither the disable state nor fault signal output is obtained when a light-emitting element and/or a circuit for driving that light-emitting element malfunctions.
The first and second specifications can both be realized by conventional optical transceivers such as those described above. That is because the second specifications are achieved unless output #22 of the optical transceiver is taken to outside the optical transceiver. However, the fourth specifications cannot be realized. This is because disable signals appear inside the laser driver IC. The conventional laser driver IC cannot be used in the fourth specifications, since it generates disable signals inside the IC.
For this reason, difficulty arises in meeting a variety of customers' specifications, using the same laser driver IC and without increasing the number of terminals.